The present invention relates to a microphone test fixture. Specifically, the present invention relates to a microphone test fixture that removes and eliminates resonant and echoed acoustic waves from a test chamber such that the acoustic pressures are consistent throughout the test chamber.
Acoustic test chambers only work when the device under test and the reference microphone are exposed to the same acoustic pressure. Acoustic resonances due to standing waves in the cavity prevent uniform pressures in the chamber, the frequency at which these non-uniform pressure fields form are dependent on the dimensions and design of the test fixture. These non-uniform pressure fields prevent accurate and repeatable measurements of the acoustic environment.
A typical microphone test chamber has an acoustic source, a test chamber (with all boundaries possessing infinite impedances), and a reference microphone (to determine the pressure that the device under test is experiencing). At least one dimension of the test site, and usually all of the dimensions, is/are longer than the wavelength of sound being measured.
At low frequencies, the cavity size is generally very small compared to the wavelength of the acoustic pressure being tested. The wave travels and reflects the off the walls, but the wavelength prevents the perfect cancelation of the reflected wave.
This is illustrated in FIG. 1. A test chamber 100 includes an acoustic source 105 (e.g., a speaker), a device under test (DUT) 110 (i.e., a microphone), and a reference microphone 115. The consistent shading inside the chamber 100 indicates the acoustic pressure is equal throughout the chamber 100.
However, at higher frequencies, where the wavelength of the acoustic pressure is smaller than the cavity size, the acoustic pressure throughout the cavity is no longer equal. As shown in FIG. 2, acoustic energy leaves the speaker 105 and reflects off the rigid cavity walls, and resonances in the cavity 100 occur. The result is the pressure at the microphone 110 opening is not equal to the source pressure from the speaker 105 or to the pressure at the reference microphone 115. This is indicated by the inconsistent shading throughout the chamber 100.
In the scenario shown in FIG. 2, there is a standing wave mode where there is near zero pressure in the middle of the test chamber 100, and a maximum pressure at the acoustic source 105 and the reference microphone 115. At even higher frequencies, additional resonances occur.